1. Field of the Invention
This invention relates to a treatment chair whose backrest is designed to be raised and lowered at will with respect to a seat of the chair and which is suitable for use as a treatment chair for dental, medical treatment or the like purposes or for use as a barber's chair, and more particularly to improvements in the prior art treatment chair disclosed in Japanese Utility Model Publication No. 623/1965 (U.S. Pat. No. 3,284,135, DAS No. 1248860).
2. Prior Art
Referring to the characteristic features of said prior art treatment chair P with reference to FIG. 1 showing a side view of the chair P wherein, for convenience sake, the same reference characters are used as for the corresponding members of the present invention, when a lower end of a backrest frame 2 was drawn by a driving mechanism 6 in the direction of a seat fram 1, the backrest fram 2 was tilted with respect to the seat frame 1 and, simultaneously therewith, the lower end of the frame 2 dipped in an arcuate movement, and accordingly the head and back regions of a patient were prevented from sliding along the backrest frame 2 during the tilting of the backrest frame 2. This was the feature of the previous invention P. In the treatment chair P, as apparent from the above referenced specification and description of the drawings thereof, a straight groove was used as a guide groove 11, with the result that the prior art chair P was found to have the following points to be improved. (Refer also to FIG. 2 which is an operating principle diagram illustrating the rise and fall movement of the backrest frame 2 of the chair in FIG. 1.) When the stroke of a piston rode 7 of a driving mechanism (hydraulic cylinder) is extended by a fixed length l, the locus of movement of the upper portion (for example, the uppermost end D of line C.sub.1 running through the center of a pin B and the center of a roller 9) represents a measure (a segment of an arc) corresponding to the tilting speed of the backrest frame 2. In other words, when the pin B of the backrest frame 2 is moved from B.sub.1 to B.sub.2 and if the stroke of the piston rod 7 of the hydraulic cylinder 6 is extended by a fixed length l, point D is moved from D.sub.1 to D.sub.2 on an arc a of a circule with a pin 3 as the center and with a segment of line 3-B as a radius. Similarly, at other angles of the backrest frame 2, B.sub.3 -B.sub.4 and D.sub.3 -D.sub.4 are found.
It may now be seen when a comparison is made between arc segment D.sub.1 -D.sub.2 and arc segment D.sub.3 -D.sub.4, the former is longer than the latter. In other words, the speed (peripheral speed) at which the backrest frame 2 is moved while at a raised position is faster than that at which the frame 2 is moved while at a lowered position. This difference in speed produces a feeling of discomfort in the patient.